Most present day energy usage is derived from the sun. This comes largely from the burning of fossil fuels. Such usage has caused vast environmental problems, starting from as early as the Industrial Revolution and continuing today at an almost unabated pace. Indeed, the subsequent neglect of these growing problems has given rise to the present-day situation, calling for rapid remediation on the scale of a few years.
Direct use of solar energy in photovoltaic (PV) and thermal systems is probably the most desirable—yet least used—of the so-called “green technologies” under consideration for overcoming environmental problems for home, industrial and/or large-scale usage. High construction costs and the difficulty of achieving high solar cell efficiencies are the principal factors in preventing the extensive use of most presently-available systems. Most efforts to reduce costs of solar systems are centered on improving the efficiency and cost of the solar cells themselves. For example, extensive work is underway on improving cells made of single crystal silicon and other PV materials (such as, for example, plastic films based on polysilicon, organic PV material, inorganic PV material, and the like). New physical properties, such as large charge multiplication and high voltage charge extraction, are also being studied. The present cost of production of the best solar PV cells is approximately $2-$5 per watt, which is prohibitive when compared with a current cost of about $0.50 per watt for coal, the least expensive of the fossil fuels. Often, government subsidies attempt to make up the difference in cost in order to promote alternative solar usage.
Most current solar systems for residential or business use are based on large arrays of planar, flat-plate solar PV panels set out on rooftops. Also, thin-film PV systems are being tested on vast stretches of desert floor for large-scale power plant use, for example. The flat-plate design requires that the active area of the collector be essentially equal to the area of the PV material exposed to one sun radiation. That is, there is a one-to-one ratio of active collector area to PV cell area. The production cost of a one-sun flat plate module is mostly governed by the cost and efficiency of the PV material that is used to cover the active module area. Therefore, in order to reduce the cost of a flat-plate module, the PV base material must be made less expensive, or more efficient, or both. Many organizations are investigating thin-film photovoltaic technologies to address the issue of lowering the cost of the PV base material. All thin-film approaches thus far have lowered the cost of the PV material, but at the expense of module efficiency.
Other approaches to achieve solar generation involve use of solar concentrating systems. These systems generally use parabolic mirror collectors or Fresnel lenses in various configurations to focus and concentrate the sun's light onto small-area PV cells, or fluid-filled thermal absorbers for driving turbines or other heat-generating systems. The concentration ratio is defined as the input power: output power and in these designs may vary from 1.5:1 to over 1500:1. Traditionally, design approaches for concentrating collectors have been large and bulky, using Fresnel lenses or large area parabolic reflectors. These arrangements have large single-element focusing optics, requiring highly accurate and expensive feedback-controlled solar tracking mechanisms.
One exemplary prior art solar concentrator that addresses some of these concerns is described in U.S. Pat. No. 6,276,359 issued to S. Frazier on Aug. 21, 2001. The Frazier arrangement comprises a “double reflecting” solar concentrator that utilizes a primary parabolic reflective surface in combination with a secondary reflective surface. The incident light reflects off the secondary surface away from the primary parabolic surface's natural focus point toward a second focal point positioned on (or substantially near) the surface of the primary parabolic reflective surface. This optical path results in a narrower field of view at the receiver, which can improve the costs of some PV arrangements. The high cost of the pruir arrangement is due to the small acceptance angle, however, remains limited in terms of the angle of acceptance of the incoming radiation and the need to accurately track the movement of the sun to provide a practical arrangement.
U.S. Pat. No. 6,666,207 issued on Dec. 23, 2003 to E. Arkas et al. discloses a solar concentrator formed into the shape of a spiral horn, where the horn is adapted to concentrate, by multiple reflections from the internal light-reflecting surface of the horn, solar energy incident within a predetermined range of angles. In particular, a preferred embodiment of the Arkas et al. design utilizes a spiral horn having the geometry of the well-known “Golden Spiral”. While potentially interesting from a design point of view, the formation of such a spiral horn had extensive manufacturing difficulties which make it a cost-prohibitive option.
Long parabolic troughs are used in many conventional solar collector systems, where only the elevation is feedback-controlled (that is, azimuthal control is not a concern). The design of such a trough system is based on a technique called “non-imaging optics”. This type of analysis considers principally the power concentration features of solar collectors and totally neglects the imaging features which can often be complex and highly aberrated. Also, the large physical size of the solar trough systems is a major contribution to the high cost of such arrangements.
The state of the art approaches have not adequately addressed the issues of optical efficiency, optical cost, heat dissipation, solar tracking tolerance and size and weight concerns. Although interest in solar energy usage is high, experts predict it will take years (varying from a few years to a few decades) and large investments of capital and possibly government subsidies to significantly reduce our dependence on fossil fuels.